Monday 4 October 2010

LowerMyRights

Last year I posted about some techniques and tools to restrict the rights of applications on Windows XP when you run with admin rights. I mentioned a new tool, LowerMyRights, which I forgot to publish. So here it is.

You would use LowerMyRights.dll only if the other tools and techniques are not appropriate for your specific case. LowerMyRights is useful when you can’t create a new process with restricted rights, but when you’ve to restrict the rights of an existing process.

When this DLL is loaded inside an existing process, it will check a whitelist and a blacklist to decide if it has to restrict the process’ rights (it also checks if it’s running on Windows XP). If the application’s name if found in the blacklist and not in the whitelist, LowerMyRights will do its job.

First, it will remove all the privileges of the primary token, except the SEChangeNotifyPrivilege.

Second, it will create a restricted token (with ACLs denying Administrator and Power Users rights) and use this token for impersonation (it uses impersonation because Windows doesn’t allow modifications to the ACLs of a primary token).

This impersonation is also a weak point of LowerMyRights compared with the other tools: exploit code can switch back to the unrestricted primary token by calling RevertToSelf.

You can load LowerMyRights inside all processes by adding it to the AppInit_DLL registry key, but be careful, this might cripple your system as it is loaded inside every process (even at boot time), so please test first.
Or else you use LoadDLLViaAppInit, or add it to the import table like explained here.

The whitelist (lowermyrights.wl.txt) is just a text file with a list of applications to whitelist (i.e. not lower the rights). You must use full pathnames in the whitelist.
The blacklist (lowermyrights.bl.txt) is just a text file with a list of applications to blacklist (i.e. to lower the rights). You must not use full pathnames in the whitelist, but just the application’s name.
The idea I had with this different operation of the whitelist and blacklist, is that you would be able to whitelist specific applications while blacklisting copies/fakes of these applications.
An example with notepad will make this clear: by adding c:\windows\system32\notepad.exe to the whitelist and notepad.exe to the blacklist, you would be able to use the original notepad.exe with full rights, while copies of notepad (located at other locations) or other programs with the name notepad.exe would be restricted. With hindsight, I don’t think this dual list feature is useful, but I left it in anyways (the program is a year old, I used it for a year and I haven’t modified it).

[…] LowerMyRights – didierstevens.com When this DLL is loaded inside an existing process, it will check a whitelist and a blacklist to decide if it has to restrict the process’ rights (it also checks if it’s running on Windows XP). […]

[…] LowerMyRights – didierstevens.com When this DLL is loaded inside an existing process, it will check a whitelist and a blacklist to decide if it has to restrict the process’ rights (it also checks if it’s running on Windows XP). […]